This invention relates generally to the loading and unloading of gas compressors by controlling the effective volume of the working space in the compressor cylinder chamber, or compression chamber.
More particularly, this invention concerns a method and apparatus for maintaining valved control of communication between a cylinder chamber, and an unloader chamber which may be placed in communication with the working space of the cylinder chamber.
A wide variety of gases (including air, natural gas, and other gaseous materials) are often compressed by means of a compressor. Often the compressor takes the form of a reciprocating compressor. A portion of such a compressor is illustrated schematically at 10 in FIG. 1.
Such compressors 10 have one or more cylinders 12, with a suction valve 51, and a discharge valve 52 in or near one end of a cylinder 12. A piston 53 moves back and forth, or reciprocates, in the cylinder 12. The working space within the cylinder 12 defined by the piston 53 and the cylinder head 50 is referred to as the cylinder chamber 11.
When the piston 53 moves away from the cylinder head 50 (downwardly in FIG. 1), the piston 53 makes a suction stroke; the suction valve 51 is open; the discharge valve 52 is closed; and gas is drawn into the cylinder chamber 11 as the piston 53 moves away from the cylinder head 50.
When the piston 53 moves toward the cylinder head 50 (upwardly in FIG. 1), the piston 53 makes a compression stroke; the suction valve 51 is closed; the piston 53 moves toward the cylinder head 50 to compress gas in the cylinder chamber 11. The discharge valve 52 opens and the piston 53 urges a portion of the compressed gas out of the cylinder chamber 11 into the compressed gas line, manifold or header 54, which may be generically referred to as the "discharge header" 54.
The output pressure from the cylinder chamber 11 is a function of the compression ratio. The compression ratio may be defined as the ratio of the volume of compressible gas at the end of the suction stroke (when the piston 53 is farthest removed from the cylinder head 50), to the volume of compressible gas at the end of the compression stroke (when the piston 53 is closest to the cylinder head 50). The volume of compressible gas includes the gas in the cylinder chamber 11 and any gas that is in communication with the cylinder chamber. If additional volumes of gas are placed into communication with a given cylinder chamber 11, the effective volume of compressible gas will be enlarged.
For a given size cylinder 12 and piston 53, both the compression ratio and the driver load are high if the volume of the cylinder chamber 11 when the piston 53 is closest to the cylinder head 50 at the end of the compression stroke is very small. If the effective volume of the cylinder chamber 11 is enlarged, the compression ratio is reduced, the actual compression effected on each compression stroke is reduced, and the driver load on the compressor 10 is reduced.
One way of enlarging the effective volume of the cylinder chamber 11 is to provide an unloader chamber 13, which may be selectively placed into communication with the cylinder chamber 11 by means of an unloader valve 55. When the unloader valve 55 is open, the effective volume of the cylinder chamber 11 is enlarged, and the compression ratio and the driver load of the compressor 10 are both reduced. When the unloader valve 55 is closed, the effective volume of the cylinder chamber 11 is smaller and the compressor load is at its maximum.
A compressor unloading mechanism of the type shown schematically in FIG. 1 is useful when it is desired to have a gas compressor driver operate at rated load--which often is less than maximum full capacity. This invention relates to an improved means for maintaining valved control of communication between the cylinder chamber and the unloader chamber.
In the past, externally controlled unloader valves have been used which require a pressure regulator to step down gas pressure from the compressor output levels typically of 600 psi to a suitable control level of typically 125 psi. Other prior art devices have employed pressurized fluids which require external sources of pressure. Such devices have been unsatisfactory in that the relatively low control pressure must operate against the relatively high pressure in the compressor cylinder head chamber in order to close communication between the cylinder chamber and the unloader chamber.
Scheerer's U.S. Pat. No. 2,833,462 illustrate a prior art device wherein pressurized fluid must be introduced into a cylinder to move a hollow piston into a closed position to reduce the effective volume of the compressor cylinder head chamber. Scheerer requires an external source of pressurized fluid. The pressurized fluid, which is used to move the hollow piston, must work against the relatively high pressure in the cylinder head chamber.
Other prior art references of interest include U.S. Pat. Nos. 4,068,562; 2,261,911; 2,241,195; 1,588,257; 3,972,652; 1,621,913; 1,587,015; 2,761,615.
The features of the prior art devices which are discussed above are not intended to be exhaustive. Other features and problems exist. The above discussion does indicate that prior art devices have left room for significant and needed improvement.